This invention relates to a planetary gear transmission apparatus and particularly to a planetary reduction gear that is able to reduce high speed motor rotation and to increase torque.
Motor rotation transmission usually uses a gear set for reducing or increasing rotation speed. The planetary gear transmission has high variable speed ration and is widely used in many different type of mechanical transmission application.
FIGS. 1 and 2 illustrate a conventional gear transmission device 10 which includes a sun gear 11, a first inner gear ring 12, a second inner gear ring 13 and a planetary gear set 14 consisting of a plurality of planetary gears 141.
The first and second inner gear ring 12 and 13 have respectively a plurality of teeth 121 and 131 formed therein. The first inner gear ring 12 has a circular space with an inside diameter slightly larger than the outside diameter of the second inner gear ring 13 for housing the second inner gear ring 13 therein and forming a hollow compartment within the first and second inner gear ring. The planetary gear set 14 has a pair of planetary gears 141 each being held between a pair of gear racks 142 by a pin 143, and engages respectively with the first and second inner gear ring 12 and 13. The assembled planetary gear set 14 is held in the hollow compartment. When in use, a sun gear 11 engages with the planetary gears 141 at a left half portion thereof which in turn engages with the first inner gear ring 12 (shown in FIG. 2) for driving the planetary gears 141. As the first inner gear 12 is usually serving as a power input ring, the rotating planetary gears 141 then drive the second inner gear ring 13 rotating for output power at reduced speed. The teeth number of the first inner gear ring 12 usually is less than the second inner gear ring 13. Hence the crown and root circles of the first inner gear ring 12 are smaller than those of the second inner gear ring 13. For the planetary gears 141 to engage both with the first and second inner gear rings 12 and 13, the teeth profile of the planetary gears 141 should be made in a step manner to match different size of crown and root circles. For instance, the left half portion of the planetary gears 141 has a lower teeth profile for engaging with the first inner gear ring 12 while the right half portion has a higher teeth profile for engaging with the second inner gear ring 13. The design and fabrication of the planetary gears 141 thus become more complicated and expensive. The high profile teeth has narrower teeth crown which tends to produce noise and is prone to rupture.
Furthermore, the sun gear 11 engages with only a half section of the axial length of the planetary gears 141. The axial positioning and alignment between the gears is difficult to attain at precise level and may result in vibration and noise production. The high speed rotation also tends to create vibration result from torque and may result in high friction and temperature, lower transmission efficiency, accelerated wearing and lower durability.
Moreover heat treatment during fabrication of the planetary gears 141 will have high stress concentration happened at the comer section like the juncture of step teeth which may result in teeth rupture under high speed or high work load condition.
In order to remedy the problems set forth above, the components of the gear transmission device 10 have to be made of higher strength steel at a greater size. Hence it is mostly used in large size transmission means such as premixture concrete truck, heavy truck and the like. It rarely being used in smaller size apparatus such as electric power hand-tools in which gear transmission means are mostly made by lower cost powder metallurgy technique.
There is another problem for the conventional transmission device 10. The friction contact (other than teeth bearing surface) between the rotating elements is usually plane contact which has high friction coefficient. A slight deviation between the contact surface may result in vibration. Friction induced temperature is high and easily reaches above 100xc2x0 C., and may result in poorer lubrication effect. Hence transmission efficiency of the conventional gear transmission device is usually lower than 0.9. There is still a room for improvement.
It is an object of this invention to provide a planetary gear transmission apparatus, which is lower cost to fabricate and assemble, has lower noise during operation, has longer durability and may be used in small size devices.
It is another object of this invention to provide a planetary gear transmission apparatus that has lower friction and operation temperature and enhanced transmission efficiency.
In one aspect, the transmission apparatus of this invention includes a sun gear, a first inner gear ring, a second inner gear ring and a planetary gear set consisting of a plurality of planetary gears. The first inner gear ring has an increased tooth root circle, larger tooth space and smaller teeth thickness. The second inner gear ring has a reduced tooth root circle, smaller tooth space and larger tooth thickness. The first and second inner gear rings have substantially same size of tooth root circle. The planetary gear has uniform teeth profile on a barrel type member. Hence stress concentration may be avoided. The apparatus may be made smaller size at lower cost. The friction contact surface between the rotation elements are formed in a curved plane so that friction loss and heat may be reduced.
In another aspect, the teeth of the inner gear ring is fabricated by gear rack cutting. For the first inner gear ring, the tooth crown height is increased by about 20-35% than regular value (e.g., the value for making the conventional tooth as which illustrated in FIGS. 1 and 2). The tooth root height is reduced by about 10-25% and the tooth thickness is increased by about 2-10% than regular value. For the second inner gear ring, the tooth crown height is reduced by about 1-5% tooth root height is increased by about 1-5% and tooth thickness is reduced by about 1-8% than regular value.
Each planetary gear is held between a pair of gear racks by a pin with about 0.1-0.15 mm of tolerance between the gear and the rack for the gear to rotate smoothly between the racks. The contact surface between the gear ring and the rack has a curvature of about 20-50 times of the gear diameter.